Damp area in 2016 Symbol

Hi,  just had habitation report for our 2016 AS Symbol. The engineer was baffled to find high damp readings in a line from centre rear of front  passenger seat and increasing as he moved towards rear of vehicle. The peak (40 to 70!) was found central to the floor area under the rear passenger seat. Then dropped quickly to 16 under the wardrobe, adjacent to rear of shower. None of the other readings around edges and joints showed anything unusual. Any ideas what the source of this damp could be?